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diff --git a/doc/src/objectmodel/properties.qdoc b/doc/src/objectmodel/properties.qdoc new file mode 100644 index 0000000..3708a2a --- /dev/null +++ b/doc/src/objectmodel/properties.qdoc @@ -0,0 +1,278 @@ +/**************************************************************************** +** +** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies). +** Contact: Nokia Corporation (qt-info@nokia.com) +** +** This file is part of the documentation of the Qt Toolkit. +** +** $QT_BEGIN_LICENSE:LGPL$ +** No Commercial Usage +** This file contains pre-release code and may not be distributed. +** You may use this file in accordance with the terms and conditions +** contained in the Technology Preview License Agreement accompanying +** this package. +** +** GNU Lesser General Public License Usage +** Alternatively, this file may be used under the terms of the GNU Lesser +** General Public License version 2.1 as published by the Free Software +** Foundation and appearing in the file LICENSE.LGPL included in the +** packaging of this file. Please review the following information to +** ensure the GNU Lesser General Public License version 2.1 requirements +** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html. +** +** In addition, as a special exception, Nokia gives you certain +** additional rights. These rights are described in the Nokia Qt LGPL +** Exception version 1.1, included in the file LGPL_EXCEPTION.txt in this +** package. +** +** If you have questions regarding the use of this file, please contact +** Nokia at qt-info@nokia.com. +** +** +** +** +** +** +** +** +** $QT_END_LICENSE$ +** +****************************************************************************/ + +/*! + \page properties.html + \title Qt's Property System + \brief An overview of Qt's property system. + + Qt provides a sophisticated property system similar to the ones + supplied by some compiler vendors. However, as a compiler- and + platform-independent library, Qt does not rely on non-standard + compiler features like \c __property or \c [property]. The Qt + solution works with \e any standard C++ compiler on every platform + Qt supports. It is based on the \l {Meta-Object System} that also + provides inter-object communication via \l{signals and slots}. + + \section1 Requirements for Declaring Properties + + To declare a property, use the \l {Q_PROPERTY()} {Q_PROPERTY()} + macro in a class that inherits QObject. + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 0 + + Here are some typical examples of property declarations taken from + class QWidget. + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 1 + + A property behaves like a class data member, but it has additional + features accessible through the \l {Meta-Object System}. + + \list + + \o A \c READ accessor function is required. It is for reading the + property value. Ideally, a const function is used for this purpose, + and it must return either the property's type or a pointer or + reference to that type. e.g., QWidget::focus is a read-only property + with \c READ function, QWidget::hasFocus(). + + \o A \c WRITE accessor function is optional. It is for setting the + property value. It must return void and must take exactly one + argument, either of the property's type or a pointer or reference + to that type. e.g., QWidget::enabled has the \c WRITE function + QWidget::setEnabled(). Read-only properties do not need \c WRITE + functions. e.g., QWidget::focus has no \c WRITE function. + + \o A \c RESET function is optional. It is for setting the property + back to its context specific default value. e.g., QWidget::cursor + has the typical \c READ and \c WRITE functions, QWidget::cursor() + and QWidget::setCursor(), and it also has a \c RESET function, + QWidget::unsetCursor(), since no call to QWidget::setCursor() can + mean \e {reset to the context specific cursor}. The \c RESET + function must return void and take no parameters. + + \o A \c NOTIFY signal is optional. If defined, the signal will be + emitted whenever the value of the property changes. The signal must + take one parameter, which must be of the same type as the property; the + parameter will take the new value of the property. + + \o The \c DESIGNABLE attribute indicates whether the property + should be visible in the property editor of GUI design tool (e.g., + \l {Qt Designer}). Most properties are \c DESIGNABLE (default + true). Instead of true or false, you can specify a boolean + member function. + + \o The \c SCRIPTABLE attribute indicates whether this property + should be accessible by a scripting engine (default true). + Instead of true or false, you can specify a boolean member + function. + + \o The \c STORED attribute indicates whether the property should + be thought of as existing on its own or as depending on other + values. It also indicates whether the property value must be saved + when storing the object's state. Most properties are \c STORED + (default true), but e.g., QWidget::minimumWidth() has \c STORED + false, because its value is just taken from the width component + of property QWidget::minimumSize(), which is a QSize. + + \o The \c USER attribute indicates whether the property is + designated as the user-facing or user-editable property for the + class. Normally, there is only one \c USER property per class + (default false). e.g., QAbstractButton::checked is the user + editable property for (checkable) buttons. Note that QItemDelegate + gets and sets a widget's \c USER property. + + \o The presence of the \c CONSTANT attibute indicates that the property + value is constant. For a given object instance, the READ method of a + constant property must return the same value every time it is called. This + constant value may be different for different instances of the object. A + constant property cannot have a WRITE method or a NOTIFY signal. + + \o The presence of the \c FINAL attribute indicates that the property + will not be overridden by a derived class. This can be used for performance + optimizations in some cases, but is not enforced by moc. Care must be taken + never to override a \c FINAL property. + + \endlist + + The \c READ, \c WRITE, and \c RESET functions can be inherited. + They can also be virtual. When they are inherited in classes where + multiple inheritance is used, they must come from the first + inherited class. + + The property type can be any type supported by QVariant, or it can + be a user-defined type. In this example, class QDate is considered + to be a user-defined type. + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 2 + + Because QDate is user-defined, you must include the \c{<QDate>} + header file with the property declaration. + + For QMap, QList, and QValueList properties, the property value is + a QVariant whose value is the entire list or map. Note that the + Q_PROPERTY string cannot contain commas, because commas separate + macro arguments. Therefore, you must use \c QMap as the property + type instead of \c QMap<QString,QVariant>. For consistency, also + use \c QList and \c QValueList instead of \c QList<QVariant> and + \c QValueList<QVariant>. + + \section1 Reading and Writing Properties with the Meta-Object System + + A property can be read and written using the generic functions + QObject::property() and QObject::setProperty(), without knowing + anything about the owning class except the property's name. In + the code snippet below, the call to QAbstractButton::setDown() and + the call to QObject::setProperty() both set property "down". + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 3 + + Accessing a property through its \c WRITE accessor is the better + of the two, because it is faster and gives better diagnostics at + compile time, but setting the property this way requires that you + know about the class at compile time. Accessing properties by name + lets you access classes you don't know about at compile time. You + can \e discover a class's properties at run time by querying its + QObject, QMetaObject, and \l {QMetaProperty} {QMetaProperties}. + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 4 + + In the above snippet, QMetaObject::property() is used to get \l + {QMetaProperty} {metadata} about each property defined in some + unknown class. The property name is fetched from the metadata and + passed to QObject::property() to get the \l {QVariant} {value} of + the property in the current \l {QObject}{object}. + + \section1 A Simple Example + + Suppose we have a class MyClass, which is derived from QObject and + which uses the Q_OBJECT macro in its private section. We want to + declare a property in MyClass to keep track of a priorty + value. The name of the property will be \e priority, and its type + will be an enumeration type named \e Priority, which is defined in + MyClass. + + We declare the property with the Q_PROPERTY() macro in the private + section of the class. The required \c READ function is named \c + priority, and we include a \c WRITE function named \c setPriority. + The enumeration type must be registered with the \l {Meta-Object + System} using the Q_ENUMS() macro. Registering an enumeration type + makes the enumerator names available for use in calls to + QObject::setProperty(). We must also provide our own declarations + for the \c READ and \c WRITE functions. The declaration of MyClass + then might look like this: + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 5 + + The \c READ function is const and returns the property type. The + \c WRITE function returns void and has exactly one parameter of + the property type. The meta-object compiler enforces these + requirements. + + Given a pointer to an instance of MyClass or a pointer to an + instance of QObject that happens to be an instance of MyClass, we + have two ways to set its priority property. + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 6 + + In the example, the enumeration type used for the property type + was locally declared in MyClass. Had it been declared in another + class, its fully qualified name (i.e., OtherClass::Priority) would + be required. In addition, that other class must also inherit + QObject and register the enum type using Q_ENUMS(). + + A similar macro, Q_FLAGS(), is also available. Like Q_ENUMS(), it + registers an enumeration type, but it marks the type as being a + set of \e flags, i.e. values that can be OR'd together. An I/O + class might have enumeration values \c Read and \c Write and then + QObject::setProperty() could accept \c{Read | Write}. Q_FLAGS() + should be used to register this enumeration type. + + \section1 Dynamic Properties + + QObject::setProperty() can also be used to add \e new properties + to an instance of a class at runtime. When it is called with a + name and a value, if a property with the given name exists in the + QObject, and if the given value is compatible with the property's + type, the value is stored in the property, and true is returned. + If the value is \e not compatible with the property's type, the + property is \e not changed, and false is returned. But if the + property with the given name doesn't exist in the QObject (i.e., + if it wasn't declared with Q_PROPERTY(), a new property with the + given name and value is automatically added to the QObject, but + false is still returned. This means that a return of false can't + be used to determine whether a particular property was actually + set, unless you know in advance that the property already exists + in the QObject. + + Note that \e dynamic properties are added on a per instance basis, + i.e., they are added to QObject, not QMetaObject. A property can + be removed from an instance by passing the property name and an + invalid QVariant value to QObject::setProperty(). The default + constructor for QVariant constructs an invalid QVariant. + + Dynamic properties can be queried with QObject::property(), just + like properties declared at compile time with Q_PROPERTY(). + + \sa {Meta-Object System}, {Signals and Slots} + + \section1 Properties and Custom Types + + Custom types used by properties need to be registered using the + Q_DECLARE_METATYPE() macro so that their values can be stored in + QVariant objects. This makes them suitable for use with both + static properties declared using the Q_PROPERTY() macro in class + definitions and dynamic properties created at run-time. + + \sa Q_DECLARE_METATYPE(), QMetaType, QVariant + + \section1 Adding Additional Information to a Class + + Connected to the property system is an additional macro, + Q_CLASSINFO(), that can be used to attach additional + \e{name}--\e{value} pairs to a class's meta-object, for example: + + \snippet doc/src/snippets/code/doc_src_properties.qdoc 7 + + Like other meta-data, class information is accessible at run-time + through the meta-object; see QMetaObject::classInfo() for details. +*/ |